1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device incorporating a circuit built with thin-film transistors (hereinafter TFTs), and also relates to an apparatus for inspecting a semiconductor.
2. Description of the Related Art
TFTs using polycrystalline silicon film are being eagerly researched and developed for higher performance by different manufacturers with intense competition among them. In progress are various kinds of uses of such TFTs with a view to achieving, for example, sheet computers and the like in the future. One way of obtaining higher performance with liquid crystal display devices is to increase the mobility of carriers in TFTs. For this purpose, polycrystalline silicon film is superseding amorphous silicon film for application in TFTs. Since, in TFTs using polycrystalline silicon film, the performance and quality of the polycrystalline silicon film is the key to enhanced TFT characteristics, it is becoming increasingly necessary to develop a reliable fabrication method that ensures higher performance and quality in polycrystalline silicon films.
TFTs using polycrystalline silicon film are fabricated, for example, by first forming an amorphous silicon film on a glass substrate by a chemical vapor growth method and then modifying the amorphous silicon film into polycrystalline silicon film. One way of achieving crystallization is by laser processing, whereby the amorphous silicon film on the glass substrate is irradiated with an excimer laser so as to be modified into polycrystalline silicon film. Crystallization by laser processing can be performed at a lower temperature than the strain point of the glass substrate, and thus offers the advantage of omitting the use of an expensive heat-resistant quartz substrate. This is inviting intense competition in the development of crystallization by laser processing among a number of academic institutions and industrial enterprises. Moreover, for further enhanced TFT characteristics, techniques for obtaining larger grain size in crystallization by laser processing are being developed.
One conventionally developed technique for obtaining larger grain size in crystallization by laser processing is a method called the CGS (continuous grain silicon) method, whereby polycrystalline silicon film is formed by the use of a catalyst. The applicant of the present invention is the pioneer in developing this method. According to this method, a metal element that promotes crystallization (for example, nickel) is added to amorphous silicon film, which is then heated so that crystalline silicon starts forming from regions where the metal element has been added (the SPC step), followed by laser processing whereby crystallization is spread over substantially the entire area of the amorphous silicon film. By this method, it is possible to produce high-performance polycrystalline silicon with a carrier mobility of about 100 cm2/Vs or more.
Among other conventionally developed techniques for obtaining a larger grain size is one involving laser processing performed in a plurality of steps.
Crystallization by laser processing, however, has the following disadvantages. As crystallization progresses, grain boundaries with a grain size of, for example, about 1 μm or more form in the polycrystalline silicon film. As the grain size increases, projections grow in the polycrystalline silicon film along the grain boundaries in such a way as to push up the enlarged crystal grains, increasing the surface irregularities (surface roughness) of the polycrystalline silicon film.
Moreover, if laser processing is performed at a laser power far lower than the optimum laser power, the amorphous silicon does not completely crystallize. At a higher laser power, crystallization progresses, but a laser power even slightly higher than the optimum laser power value promotes re-crystallization of the amorphous silicon film, resulting in poor characteristics in the polycrystalline silicon film.
Moreover, the laser power of laser processing equipment varies with time from the initially set level, and accordingly the quality of the amorphous silicon film on the glass substrate varies with time. This hampers stable formation of the polycrystalline silicon film.
As discussed above, polycrystalline silicon film produced by laser processing is sensitive to the laser power at which it is processed. This makes it essential to set a proper laser power.
For this reason, there have conventionally been disclosed and proposed monitoring methods according to which a fabrication process additionally includes a step of determining the optimum laser power value so that a laser processing step is performed with the thus determined optimum laser power value (for example, see Japanese Patent Application Laid-Open No. 2001-257176).
With a method for fabricating a semiconductor device that additionally includes a step as described above, it may be possible to set the optimum laser power value to a certain extent.
However, no monitoring methods have been conventionally described that permit setting the optimum laser power for substrates that have undergone preprocessing involving one or more steps for modifying amorphous silicon film. Thus, there is a possibility that the fabrication process that is used to produce monitor substrates (e.g., test substrates) in the course of which the optimum laser power is determined differs from the fabrication process that is used to produce product substrates in the course of which laser processing is performed at the thus determined laser power. For example, as shown in the process flow chart in FIG. 1, there is a possibility that, for a monitor substrate with amorphous silicon film that has not undergone an SPC step 2 in which it would be heated with a catalyst added thereto, the laser power is determined in an optimum power inspection/extraction step 4.
Moreover, no detailed description has conventionally been given of how to evaluate the results obtained in the optimum power inspection/extraction step 4 to determine the optimum laser power, nor how to make evaluations on an absolute basis for automatic approval and rejection.
As discussed above, in practice, a laser processing step is not always performed at the optimized laser power, resulting in amorphous silicon film being produced with poor performance and quality. This makes it necessary to set a more optimal laser power in producing polycrystalline silicon film.